This application claims the priority of PCT International Application No. PCT/EP00/05803 filed Jun. 23, 2000 and German patent document 199 39 817.8, filed Aug. 21, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for automatic braking operation of a vehicle
Automatic brake systems are known which trigger an automatic braking operation with a braking pressure which is greater than the braking pressure corresponding to the brake pedal position, as a function of xe2x80x9cpanic movement of the driverxe2x80x9d. Panic movement of the driver is usually detected by means of the speed of the brake pedal, and is assumed to occur if the speed of the pedal exceeds a reference value. In this case, it is assumed that the driver desires a greater braking force than he prescribes at a given time by means of the activation of the brake pedal.
Such systems, which are known under the term xe2x80x9cbrake assistantxe2x80x9d have been described, for example, in German patent documents DE 44 22 664 A1, DE 196 29 229 A1 and DE 196 41 470 A1.
The brake assistant is generally activated until one-of the brake pedal state variables again falls below the reference value, for example as a result of the driver allowing the brake pedal to return in the direction of the home position, completely releasing the brake pedal or at least reducing the brake pedal force. In this case, it is assumed that the driver desires a reduction of the braking force to a value which corresponds to the current brake pedal position.
Because the driver""s reactions which trigger an automatic braking operation are sensed by means of sensors, there is a risk that design-related signal falsifications of the sensors or incorrect signals as a consequence of sensor errors may cause the brake assistant to be triggered too early or too late, or even not at all. In particular, when a sensor containing a resistance potentiometer is used to sense brake pedal travel, design-related slider contact resistances may occur which can lead to signal notches at the input of a control device of the vehicle. Such notches can, however, usually be compensated again within a short time so that a correct signal value is present at the control device. However, the signal""s rising again after the notch may be interpreted in the control unit as panic braking and lead to triggering of an automatic, but undesired braking operation.
German patent document DE 197 22 532 A1 also discloses a method for suppressing faults at the diaphragm travel sensor of a brake booster. Here, the beginning and the end of the fault are sensed and the measure to be carried out as a result of the fault is selected as a function of the duration of the fault. The state variable sensed at the beginning of the fault is retained and remains constant to the end of the fault. In order to prevent improper triggering of an automatic braking operation at the end of the fault, the unfiltered speed value zero is fed to a speed filter.
One object of the invention is to provide a reliably functioning automatic brake system.
This and other objects and advantages are achieved by the automatic braking system according to the invention, in which if a state variable which is sensed by a sensor lies outside an acceptable tolerance range, triggering of an automatic braking operation is prevented for a specific time period. The reference value which is used as a basis for triggering an automatic braking operation expediently lies outside the tolerance range which needs to be taken into account for the suspension. This provides the advantage that malfunctions of the sensor for sensing the state variable that describes the driver""s reaction or the event, can be detected in order to trigger the brake assistant. That is, when a value which is not relevant to the triggering of the brake assistant but which lies in an unusual or marked value range is detected, the triggering of automatic braking is prevented for a specific time period. Within this time period, the automatic braking operation is suppressed even if the state variables which are sensed by sensor fulfil the triggering criteria for generating additional braking force. This ensures that when there is a signal fault which results, in particular, in a drop in the signal value, there is a subsequent dead time or holding time within which the signal can rise again to its correct value without triggering automatic braking. After the dead time has expired, the automatic braking operation can be initiated if the relevant conditions are fulfilled.
In the novel method according to the invention, a significant point in time is identified by reference to a signal value lying outside the acceptable tolerance range. This significant point in time is followed by the holding time within which it is not possible for the automatic braking operation to be triggered.
According to one embodiment, a variable which describes the status of the brake pedal, in particular the brake pedal position, the change in brake pedal position and/or the change in speed of the brake pedal is determined as the state variable to be examined for triggering of automatic braking. If the brake pedal state variables are taken into account, it is possible to implement a graduated strategy for the position, speed and acceleration levels by examining in chronological succession the value range firstly at the position level, then at the speed level and finally at the acceleration level.
At the position level, the sensor value is preferably examined to determine whether the brake pedal position decreases or returns in the direction of its home position. If so, the activation of automatic braking pressure is prevented until the braking operation which is currently under way is terminated. Although, the return of the brake pedal can basically also be determined by means of the speed of the brake pedal., it is advantageous, in particular in very low pedal speed ranges, to detect the change in the pedal position at the position level, because in the low speed range examination of the brake pedal position supplies more precise results than the examination of the speed derived from the pedal position.
The automatic generation of braking pressure can be suspended until the brake pedal position rises again. It is possible both for the brake pedal position to return as far as the unloaded home position and for the position to increase again from a partial braking position.
At the speed level there is expediently an examination to determine whether the speed of the brake pedal drops below an assigned limit value. If a negative value is prescribed as a limit value, automatic generation of braking pressure is advantageously suspended for a predefined or determined constant holding time. The examination at the speed level with the predefinition of a negative limit value makes it possible to detect a signal notch and subsequently to disable the triggering of the automatic generation of braking pressure. As a result, the time period in which the signal value rises strongly again to the correct value after the signal notch is gated out for the triggering of the automatic braking operation.
The speed limit value can be selected in such a way that unambiguous identification of a fault signal is possible; alternatively it is possible to distinguish a real return movement of the brake pedal. However, if the speed limit value lies inside the speed value range to be reached by the brake pedal, disabling of the automatic generation of braking pressure does not lead to any undesired results, because even a real return movement is not expected to trigger the automatic braking operation, at least within the subsequent holding time period which is generally selected to be very short.
In a preferred embodiment of the invention, deceleration of the brake pedal is examined to determine whether it exceeds an assigned deceleration limit value. If the pedal speed decreases within a time period under consideration, by a value that is larger than that which is predefined as acceptable (that is, the deceleration limit value), the sensor value lies outside an acceptable value range and automatic activation of braking pressure is prevented for a given or determined time period.